For the purpose of utilizing advantageous basic characteristics of copper, i.e., high electric conductivity and high thermal conductivity, in various industrial products, many copper alloys improved to overcome a weak point of copper, i.e., a deficiency in strength, have been manufactured. As methods for strengthening copper, known hitherto are a method of adding one or more of elements having less solubility into matrix, such as Zr, Cr, Cd and Be, to develop precipitation hardening by aging treatment, and a method of dispersing ceramic particles, which are hard to react with a copper matrix phase, into the matrix phase. The strengthening is effected by precipitates or ceramic particles which are finely dispersed in the mother phase and prevent dislocation movement responsible for a plastic deformation. The former strengthening method is disclosed in JP-A-57-9850. Of the latter strengthening method, oxide-dispersion-hardening is disclosed in JP-A-2-213433, carbide-dispersion-hardening in JP-A-1-96338, and nitride-dispersion-hardening in JP-A-60-208402, etc.
Meanwhile, addition of graphite powder or BN powder improves lubrication property, low contact resistance and seizure resistance of copper alloys. A method for manufacturing such copper alloys is disclosed in, for example, JP-A-57-123943. Addition of graphite powder is practiced to manufacture copper alloys for electric contacts, aiming at an improvement in low contact resistance and seizure resistance, and is disclosed in JP-A-62-284031.
The aforesaid precipitation-hardening copper alloy can keep the strength at temperatures of about 400.degree. C. or below, but loses its hardening ability at temperatures higher than 400.degree. C. due to pyrolysis of precipitates.
On the other hand, as dispersing methods used to manufacture a ceramic dispersion strengthening copper alloy, known hitherto are an oxide dispersion strengthening method using a powder mixing process or an internal oxidation process, and a mechanical alloying process adapted for a mixture of copper and fine ceramic powders as disclosed in JP-A-3-2338, JP-A-2-213433 and JP-A-63-83240. For the fine dispersion of ceramic particles, the internal oxidation process and the mechanical alloying process are superior. As methods for manufacturing members made of strengthening copper alloys, there has been utilized powder metallurgical methods such as thermal extrusion.
Taking copper as an example, if impurities are present in copper, those impurities cause scattering of conduction electrons, thereby deteriorating high electric conductivity and high thermal conductivity both inherent to copper. Strength of copper members is required to be increased for enabling them to be widely used as practical materials for industrial purposes, but any methods of strengthening copper by alloying necessarily deteriorate the above inherent characteristics. In a precipitation-hardening copper alloy, particularly, besides a limit in the application temperature as mentioned before, electron scattering occurs due to strains of lattices around the precipitates and changes in electron states around a trace amount of residual metallic solute atoms, making it very difficult to expect high electric conductivity and high thermal conductivity comparable to those of pure copper.
Although increasing the strength of copper while maintaining excellent characteristics thereof is quite difficult, an optimum method for solving this problem is the above-mentioned method of dispersing ceramic particles which are less reactive with copper.
Al.sub.2 O.sub.3 has been most widely employed as dispersion strengthening particles for copper, and an Al.sub.2 O.sub.3 dispersion strengthened copper alloy has been manufactured for commercial marketing. Of methods for manufacturing that strengthened copper alloy, however, an internal oxidation process of a polycrystalline powder of a copper-aluminum low alloy has the problem of deteriorating the strength at high temperatures because coarse Al.sub.2 O.sub.3 are formed at grain boundaries.
On the other hand, an Al.sub.2 O.sub.3 dispersion strengthened copper alloy manufactured by a step of mechanical alloying of mixture of copper powder and fine Al.sub.2 O.sub.3 powder, or steps of mechanical alloying to mixture of fine copper oxide powder and fine Al.sub.2 O.sub.3 powder and reducing it, and a subsequent step of sintering the alloy exhibits the superior strength. However, increasing the amount of Al.sub.2 O.sub.3 added to enhance the strength leads to an increase in the content of copper oxide in the copper matrix phase, thus resulting in deterioration of electric conductivity and thermal conductivity. This phenomenon occurs in common to those alloys using ceramic oxides as dispersion powder. As to the reasons, it is believed that during sintering of copper particles and oxide particles, an reaction occurs at contact interface therebetween to form thin copper oxide films on the copper particles, and deformations developed with progress of the sintering cause exfoliation and dispersion of the copper oxide. In the case of utilizing an oxide dispersed copper alloy as a stabilizer for metallic superconductor coils, especially, care must be paid so as to maintain a high degree of purity of copper matrix phase.
Further, during reducing heat treatment in which a large amount of copper oxide and Al.sub.2 O.sub.3 particles after the mechanical alloying are reduced at temperatures of 1065.degree. C. or below in the above manufacturing steps, the treatment temperature is hard to control because of an exothermic reaction between hydrogen and the copper oxide, making it very difficult to determine whether reduction from the copper oxide to copper has been completed or not over the entirety of powder. If the reducing reaction is incomplete, the content of oxygen remaining in the matrix would be increased, with the result of deteriorating the excellent characteristics of copper.
Ceramic dispersion particles are more or less bonded to the matrix phase, and impurities in the ceramic dispersion particles penetrate into the matrix phase during the mechanical alloying and through the interface reaction during the sintering, thereby contaminating the matrix phase. Accordingly, utilization of high purity ceramic particles is desired. It can be also said that a copper alloy in which are dispersed those ceramic particles having at least one of excellent copper characteristics, i.e., high electric and high thermal conductivities, and being hard to react with copper, is suitable as the ceramic dispersion strengthening copper alloy.
Meanwhile, in conventional copper alloys for use as wear-resistant materials, graphite or BN having superior lubrication property has been generally added. In view of practical application of graphite to electric contacts, especially, it is believed that wear-resistant contact materials exhibiting superior characteristics to conventional copper alloys can be developed by using such graphite as having excellent strength and thermal properties.